Summary of Work: Although iron and copper are essential nutrients, the pathological processes associated with the various forms of metal overload demonstrate that these metals can become toxic when high exposures occur. Examples of occupational poisoning are still reported but dominant interest now lies in the possibility of insidious effects resulting from long term exposures. Cellular injury from toxic metals may occur by a number of molecular mechanisms including metal-induced formation of free radicals. The most direct technique available for the detection of free radicals is ESR spectroscopy which for use in living systems requires a spin-trapping approach. In the study of free radical formation in vivo, we utilized the scavenging reaction in which the hydroxyl radical is converted into the methyl radical via its reaction with dimethyl sulfoxide (DMSO). The methyl radical is them detected as its long-lived phenyl N-t-butylnitrone (PBN) adduct. All of the previous studies used doses of the metals near their LD50 levels. Significantly lower doses did not give detectable radical adduct spectra. Nevertheless, we attempted to detect hydroxyl radical generation in rats with acute iron poisoning at different ages. We also studied the effect of vitamin C on free radical generation in young (2-month-old) animals and in older (6-, 9-, 12- and 24-months) rats during acute iron poisoning. We found out that iron poisoning does not cause free radical generation in young 2-month-old rats. However, simultaneous injection of 1 mmol/kg ascorbic acid and iron caused significant increase in free radical generation. In contrast, ascorbic acid did not enhance free radical generation in older rats. This was the first evidence of hydroxyl radical generation after acute iron poisoning and ascorbic acid intake in young rats. It has been postulated that the in vivo toxicity of asbestos results from its catalysis of free radical generation by surface ion. We examined in vivo radical production using ESR with the spin trap `-(4-pyridyl-1-oxide)-N-t-butylnitrone (4-POBN). We concluded that, after instillation of crocidolite in the rat, ESR analysis of lung tissue demonstrates in vivo free radical production.